This invention relates generally to micro-electro-mechanical systems (MEMS), and more specifically to MEMS electrostatic microactuators.
MEMS devices are basically xe2x80x9cmicroxe2x80x9d systems that incorporate some type of electromechanical transduction to achieve a given function. In this case, xe2x80x9cmicroxe2x80x9d refers to component features of the order of micrometers. Examples of MEMS microactuators include micropumps, micromotors, micro-optical mirrors, and micropositioners.
A typical MEMS microactuator is fabricated using semiconductor process technology and uses a flexure or spring-like structure to support a movable component or member. Microactuators can be driven using electrostatic forces, magnetic forces, thermal expansion of materials, piezoelectric effects, or surface tension forces. A device based on microactuation by surface tension of a metallic liquid is described by J. Lee et al., xe2x80x9cSurface-Tension-Driven Microactuation Based on Continuous Electrowettingxe2x80x9d, JOURNAL OF MICELECTROMECHANICAL SYSTEMS, Vol. 9, No. 2, June 2000, 171-180. A rotary type electrostatic microactuator that uses interdigitated or interleaved fingers for micro-positioning the read/write head supported on an air bearing slider in a magnetic recording disk drive is described by L-S Fan et al., xe2x80x9cElectrostatic Microactuator and Design Considerations for HDD Applicationsxe2x80x9d, IEEE TRANSACTIONS ON MAGNETICS, Vol. 35, No. 2, March 1999, 1000-1005, and also in IBM""s patent U.S. Pat. No. 5,995,334.
A common problem of microactuators, especially those that use spring-like structures to support a movable member, like electrostatic microactuators with interleaved fingers, is a relatively large vibration at a structural resonant frequency, resulting in amplification of external disturbances at that frequency. This not only severely degrades the positional accuracy of this type of microactuator but also makes difficult the closed-loop control of microactuator motion. U.S. Pat. No. 5,745,281 describes an electrostatic light modulator with a shutter that moves within a chamber containing a dielectric liquid for damping movement of the shutter.
What is needed is an electrostatic microactuator with interleaved fingers that has minimal or no resonant vibration.
The invention is a micro-electro-mechanical systems (MEMS) type electrostatic microactuator with viscous liquid damping. The MEMS substrate supports a fixed electrode and a movable electrode, with the movable electrode being attached to the substrate by a flexure. Each electrode has a plurality of fingers with the fixed electrode fingers and the movable electrode fingers interleaved in a comb-like arrangement. A nonconductive liquid is located between the fingers for damping motion of the movable electrode relative to the fixed electrode. The liquid is held in a reservoir attached to the movable electrode. Capillary pressure pulls the liquid from the reservoir into the small gaps between the interleaved fingers.
In one embodiment the reservoir is a plurality of cells, with each cell having a wall spacing greater than the gap spacing between the fingers. As a result, the capillary pressure in a partially filled reservoir cell will be less than the capillary pressure in a partially filled gap. Only when the gaps are full can the capillary pressure in the gaps equal the pressure in the partially filled reservoir, ensuring that the gaps stay full as long as liquid remains in the reservoir, even if liquid is lost over time from the microactuator. In another embodiment the gaps for different sets of adjacent fingers have different sizes so the amount of damping is controlled by changing the number of gaps with sizes less than the reservoir cell wall spacing or by changing the cell wall spacing to allow a different number of cells to be filled with liquid.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.